1. Field of the Invention
The present invention relates to a magnetostrictive torque sensor for detecting torque based on a change in a magnetic property due to magnetostriction, as well as to a method of manufacturing such a magnetostrictive torque sensor.
2. Description of the Related Art
Generally, an electric power steering apparatus on a vehicle includes a motor incorporated in a steering system, and a controller for controlling the power supplied from the motor in order to reduce a steering torque that is applied to the steering shaft by the driver of the vehicle.
As shown in FIG. 11 of the accompanying drawings, an electric power steering apparatus 200 of the related art includes a steering torque detector (torque sensor) 208 disposed in a steering gearbox 202 for detecting a steering torque, which acts on a steering shaft 206 coupled to a steering wheel 204. The torque sensor 208 outputs a detected torque value, which is supplied to a controller 236 as a reference signal, for thereby enabling a motor to generate an appropriate assistive steering torque.
More specifically, when the driver turns the steering wheel 204, the steering wheel 204 produces a steering torque and a steering angle, which are applied through the steering shaft 206 and other members to a steering shaft 210 of the steering gearbox 202.
The steering gearbox 202 comprises the steering shaft 210, the torque sensor 208, a motor 212 for generating an assistive steering torque to assist the driver in turning the steering wheel 204, a speed reducer 214 including a worm 216 and a worm wheel 218 for increasing the assistive steering torque generated by the motor 212, a rack and pinion gear mechanism 220 including a rack shaft 228 and a rack gear 222, and a screw groove 226 of a ball screw 224.
The steering wheel 204 is coupled to an end of the steering shaft 210 through the steering shaft 206 and other members. The opposite end of the steering shaft 210 forms a pinion gear 230 of the rack and pinion gear mechanism 220.
The assistive steering torque, which is increased by the speed reducer 214, is converted by the ball screw 224 into a thrust force in axial directions of the rack shaft 228. The thrust force is transmitted through tie rods 232a, 232b to left and right tires 234a, 234b of the vehicle. Depending on the steering angle of the steering wheel 204, the tires 234a, 234b turn about vertical axes thereof in order to change the direction of travel of the vehicle.
The controller 236 (ECU) controls the motor 212 based on at least the signal from the torque sensor 208, and a signal from a vehicle speed sensor 238.
The steering torque applied from the steering wheel 204 when the steering wheel 204 is turned by the driver is detected by the torque sensor 208. The controller 236 controls the motor 212 based on the torque signal from the torque sensor 208 and the signal from the vehicle speed sensor 238. The rotational torque generated by the motor 212 acts through the speed reducer 214, the ball screw 224, and the rack shaft 228 on the pinion gear 230 of the rack and pinion gear mechanism 220, thereby reducing the torque required for the driver to turn the steering wheel 204. In this manner, the burden imposed on the driver when turning the steering wheel 204, i.e., the burden imposed on the driver to produce the steering torque, is reduced.
The torque sensor 208 may be a torsion-bar torque sensor for detecting torque based on twisting of a torsion bar disposed between pinion input and output shafts, or a magnetostrictive torque sensor. In this case, it is assumed that the torque sensor 208 comprises a magnetostrictive torque sensor. A steering torque and a steering angle generated when the driver turns the steering wheel 204 are applied through couplings 240 (e.g., serrations) to the steering shaft 210 of the steering gearbox 202. As shown in FIG. 12 of the accompanying drawings, the steering shaft 210 is rotatably supported in the steering gearbox 202 by a first bearing 242a positioned near one end of the steering shaft 210, a second bearing 242b positioned centrally along the steering shaft 210, and a third bearing 242c positioned near the other end of the steering shaft 210.
Heretofore, the magnetostrictive torque sensor 208 has been made of two magnetostrictive films, i.e., a first magnetostrictive film 244a and a second magnetostrictive film 244b, which are grown on the surface of the steering shaft 210 by means of Ni—Fe plating, for example, such that the first magnetostrictive film 244a and the second magnetostrictive film 244b are disposed on respective upper and lower positions on the steering shaft 210, and have respective axial widths with mutually opposite magnetic anisotropic properties. When a steering torque is applied from the steering shaft 210 to the first magnetostrictive film 244a and the second magnetostrictive film 244b, inverse magnetostrictive properties exhibited by the first magnetostrictive film 244a and the second magnetostrictive film 244b based on the respective magnetic anisotropic properties thereof are detected based on AC resistances, etc., of a first coil 246a and a second coil 246b, which are disposed respectively around the first magnetostrictive film 244a and the second magnetostrictive film 244b, thereby detecting the steering torque. Such a magnetostrictive torque sensor 208 is disclosed in Japanese Laid-Open Patent Publication No. 2002-257648, and Japanese Laid-Open Patent Publication No. 2004-340744, for example.
Specific details of the magnetostrictive torque sensor 208 will be described below with reference to FIGS. 12 through 19 of the accompanying drawings.
The first magnetostrictive film 244a and the second magnetostrictive film 244b are formed on the steering shaft 210 by plating an Ni—Fe alloy to a thickness of about 40 μm on the steering shaft 210, which is selectively masked (see FIG. 15).
FIG. 12 shows the layout of the first coil 246a and the second coil 246b, as well as the first magnetostrictive film 244a and the second magnetostrictive film 244b of the magnetostrictive torque sensor 208. As shown in FIG. 12, the first magnetostrictive film 244a and the second magnetostrictive film 244b on the surface of the steering shaft 210 are spaced from each other by a gap 248 formed therebetween. The first coil 246a and the second coil 246b are disposed respectively around the first magnetostrictive film 244a and the second magnetostrictive film 244b, with a small gap of about 0.5 mm interposed therebetween. The first coil 246a and the second coil 246b are electrically connected to respective detecting circuits. A first back yoke 250a and a second back yoke 250b, which serve as magnetic shields, are disposed respectively over the first coil 246a and the second coil 246b. A spacer 252 is interposed between the first back yoke 250a and the second back yoke 250b. 
When a torque is applied to the steering shaft 210, the torque also is applied to the first magnetostrictive film 244a and to the second magnetostrictive film 244b, which develop an inverse magnetostrictive effect depending on the applied torque. Due to such an inverse magnetostrictive effect, the magnetic permeabilities of the first magnetostrictive film 244a and the second magnetostrictive film 244b change, resulting in changes in inductances of the first coil 246a and the second coil 246b. While the detecting circuits supply high-frequency AC voltages (exciting voltages) to the first coil 246a and the second coil 246b, changes in inductance of the first coil 246a and the second coil 246b are detected as voltage changes, which represent the applied steering torque.
An example of such inverse magnetostrictive properties is shown in FIG. 13. In FIG. 13, the horizontal axis represents the steering torque, whereas the vertical axis represents impedances of the first coil 246a and the second coil 246b, which are developed therein when AC voltages are applied to the first coil 246a and the second coil 246b. In FIG. 13, the characteristic curve C10 represents how the impedance of the first coil 246a changes, whereas the characteristic curve C11 represents how the impedance of the second coil 246b changes.
When the steering torque changes from a negative value to a positive value, the impedance of the first coil 246a increases. When the steering torque reaches a positive value T1, the impedance of the first coil 246a reaches a peak level P1, and when the steering torque has a value greater than the positive value T1, the impedance of the first coil 246a decreases. When the steering torque changes from a positive value to a negative value, the impedance of the second coil 246b or the voltage induced across the second coil 246b increases. When the steering torque reaches a negative value −T1, the impedance of the second coil 246b reaches a peak level P1, and when the steering torque increases beyond the negative value −T1, the impedance of the first coil 246a decreases. The steering torque vs. impedance characteristic curve C10 provided by the first coil 246a, as well as the steering torque vs. impedance characteristic curve C11 provided by the second coil 246b, essentially are convex upwardly oriented, as shown in FIG. 13. The steering torque vs. impedance characteristic curve C10, as well as the steering torque vs. impedance characteristic curve C11, substantially are symmetric with respect to the vertical axis due to the inverse magnetostrictive properties exhibited by the first magnetostrictive film 244a and the second magnetostrictive film 244b. FIG. 13 also shows a characteristic curve C12 representing how the impedances of the first coil 246a and the second coil 246b change when the first magnetostrictive film 244a and the second magnetostrictive film 244b are not anisotropic.
A process of rendering the first magnetostrictive film 244a and the second magnetostrictive film 244b anisotropic in order to provide the characteristic curves C10, C11 will be described below.
First, as shown in FIG. 14A, while a torque of 10 kgm is applied in one direction to the steering shaft 210, the upper first magnetostrictive film 244a is high-frequency heated to 300° C. by a heating coil 254. Thereafter, the first magnetostrictive film 244a is cooled, and then the torque is removed in order to render the first magnetostrictive film 244a anisotropic.
Then, as shown in FIG. 14B, while a torque of 10 kgm is applied in an opposite direction to the steering shaft 210, the lower second magnetostrictive film 244b is high-frequency heated to 300° C. by the heating coil 254. Thereafter, the second magnetostrictive film 244b is cooled, and then the torque is removed in order to render the second magnetostrictive film 244b anisotropic.
In FIG. 13, the straight line L10 represents values produced by subtracting the characteristic curve C11 detected by the second coil 246b from the characteristic curve C10 detected by the first coil 246a. When the steering torque is zero, the value of the straight line L10 also is zero. The straight line L10 indicates that impedance changes occur essentially rectilinearly within a torque range W in which the magnetostrictive torque sensor 208 normally is used. The magnetostrictive torque sensor 208 is used within a range in which the characteristic curves C10, C11 have a substantially constant gradient near the torque midpoint, and hence the magnetostrictive torque sensor 208 outputs a detected signal commensurate with the direction and magnitude of the applied steering torque. Based on the straight line L10, the steering torque can be detected from the impedances of the first coil 246a and the second coil 246b. 
In FIG. 13, the straight line L20 represents values produced by adding the characteristic curve C11 detected by the second coil 246b and the characteristic curve C10 detected by the first coil 246a, and then adding a certain value to the sum thereof. The values represented by the straight line L20 are constant irrespective of the steering torque. It is possible to detect failures of the detecting circuits and the first coil 246a and the second coil 246b by monitoring whether a signal generated by the magnetostrictive torque sensor 208 and represented by the straight line L20 falls within a given range or not.
With the magnetostrictive torque sensor 208, two magnetostrictive films, i.e., the first magnetostrictive film 244a and the second magnetostrictive film 244b, are disposed in respective upper and lower positions, with the gap 248 being interposed therebetween. Therefore, the magnetostrictive torque sensor 208 tends to suffer from output characteristic variations if the first magnetostrictive film 244a and the second magnetostrictive film 244b are not properly positioned or have improper lengths, or if the first coil 246a and the second coil 246b are not properly positioned.
To avoid the above problems, it has heretofore been customary to fabricate the first magnetostrictive film 244a and the second magnetostrictive film 244b so as to have a larger axial dimension than the first coil 246a and the second coil 246b, and to position the first coil 246a and the second coil 246b in alignment with respective central regions of the first magnetostrictive film 244a and the second magnetostrictive film 244b. However, since the axial dimensions of the first magnetostrictive film 244a and the second magnetostrictive film 244b must be increased, the magnetostrictive torque sensor 208, as well as the electric power steering apparatus 200 incorporating the magnetostrictive torque sensor 208 therein, inevitably become larger in size.
More specifically, the first magnetostrictive film 244a and the second magnetostrictive film 244b include regions having magnetic permeability irregularities in upper and lower end portions thereof. For example, in order to deposit the first magnetostrictive film 244a and the second magnetostrictive film 244b on the steering shaft 210 by means of Ni—Fe plating, the steering shaft 210 is masked, as shown in FIG. 15, before the steering shaft 210 is dipped into a plating solution for carrying out electrolytic plating thereon. At this time, the steering shaft 210 is selectively masked by a first mask 256a, which covers an end portion of the steering shaft 210, a second mask 256b, which covers a portion of the steering shaft 210 extending from an opposite end toward an axially central region thereof, and a third mask 256c, which covers a portion of the steering shaft 210 corresponding to the gap 248 between the first magnetostrictive film 244a and the second magnetostrictive film 244b. When the steering shaft 210 is electrolytically plated, electric lines of force are concentrated on the boundaries of the first mask 256a, the second mask 256b, and the third mask 256c on the steering shaft 210. Therefore, as shown in FIG. 16A, the electric current density, which flows in the plating process, is higher at axial upper and lower ends of the exposed regions of the steering shaft 210 where the first magnetostrictive film 244a and the second magnetostrictive film 244b are formed. As a result, as shown in FIG. 16B, the thickness of the first magnetostrictive film 244a and the second magnetostrictive film 244b at upper and lower ends thereof becomes greater than in other regions of the first magnetostrictive film 244a and the second magnetostrictive film 244b, and as a result, the first magnetostrictive film 244a and the second magnetostrictive film 244b have thickness irregularities. As shown in FIG. 17, if the first coil 246a and the second coil 246b are positioned facing the regions having thickness irregularities, then the output characteristics of the magnetostrictive torque sensor 208, i.e., a detected signal VT1 from the first coil 246a and a detected signal VT2 from the second coil 246b, are unduly changed. For example, when the steering torque is zero, the voltages (midpoint voltages) and gains of the first coil 246a and the second coil 246b, which are associated respectively with the first magnetostrictive film 244a and the second magnetostrictive film 244b, are different from each other, or the voltages (midpoint voltages) and gains of the first coil 246a and the second coil 246b, at a time when the first coil 246a and the second coil 246b face regions having thickness irregularities therein, are different from the voltages (midpoint voltages) and gains of the first coil 246a and the second coil 246b at a time when the first coil 246a and the second coil 246b do not face regions having thickness irregularities therein.
The first coil 246a and the second coil 246b tend to face the thickness irregularities when the coil housing 258 including the first coil 246a and the second coil 246b and the steering shaft 210 are secured to the steering gearbox 202, as shown in FIG. 11. At this time, the fixed positions of the first coil 246a and the second coil 246b, and the fixed positions of the first magnetostrictive film 244a and the second magnetostrictive film 244b, as well as component tolerances thereof, are likely to deviate from desired positions and tolerances.
To avoid the above problem, as described above, it has heretofore been customary to make the first magnetostrictive film 244a and the second magnetostrictive film 244b so as to have a larger axial dimension than the first coil 246a and the second coil 246b, and to position the first coil 246a and the second coil 246b in alignment with respective central regions, i.e., regions of uniform thickness, of the first magnetostrictive film 244a and the second magnetostrictive film 244b, so that the midpoint voltages and gains of the first coil 246a and the second coil 246b, which are associated respectively with the first magnetostrictive film 244a and the second magnetostrictive film 244b, remain in agreement with each other.
However, there are four regions that suffer from the above thickness irregularities, i.e., two upper and lower end portions of the first magnetostrictive film 244a, and two upper and lower portions of the second magnetostrictive film 244b. In order to position the first coil 246a and the second coil 246b out of alignment with these four regions that suffer from thickness irregularities, the axial lengths of the first magnetostrictive film 244a and the second magnetostrictive film 244b need to be increased considerably, thus resulting in an increase in size of the magnetostrictive torque sensor 208, as well as the electric power steering apparatus 200 incorporating the magnetostrictive torque sensor 208 therein. One solution would be to reduce the height or axial dimension of the gap 248 between the first magnetostrictive film 244a and the second magnetostrictive film 244b. As shown in FIG. 19, the third mask 256c that forms the gap 248 comprises a pair of laterally separate semicircular mask members 258a, 258b, having respective end faces confronting each other, and being fastened to each other by two bolts 260, which are threaded through the mask member 258b into the mask member 258a. There are certain limitations on efforts to reduce the height or axial dimension of the mask members 258a, 258b, particularly in view of the dimensions of the bolts 260.
As described above, the magnetostrictive torque sensor 208 of the related art suffers significant variations in the output characteristics (gains and midpoints) thereof, and requires a mechanism and circuit for adjusting the output characteristics. Hence, the magnetostrictive torque sensor 208 tends to be complex in structure and large in size. Furthermore, since a complex masking process is required to fabricate the first magnetostrictive film 244a and the second magnetostrictive film 244b on the steering shaft 210, the process of manufacturing the magnetostrictive torque sensor 208 also is complex, and manufacturing efficiency of the magnetostrictive torque sensor 208 cannot be increased, thereby posing limitations on efforts to lower the cost of the magnetostrictive torque sensor 208.